With respect to metallic products, there have recently been many sorts, and the high grades of qualities thereof have progressed, and accordingly observations of the solute elements are important. Almost all cases depend upon extracting samples to be analyzed and, measuring their concentrations by means of instrumental analyses such as an emission spectroscope, but a problem involved therewith was lack of speediness.
In view of such circumstances, there have been made such proposals as methods of rapidly measuring the concentrations or activities of the solute elements contained in the molten metals, at first of Japanese Patent Laid-Open 61-142,455 (Patent Laid-Open 61-260, 155, same 63-191,056, 63-286,760, 63-273,055, 63-309,849, 1-263, 556, 2-73,148, 2-82,153, Utility Model Laid-Open 63-109,643, and same 63-148,867). Basically, these conventional methods immerse, into the molten metal, a probe constituted by forming a coated layer made of an oxide (YOa) of a solute element (Y) or a composite oxide containing said oxide (YOa) on the outer surface of a solid electrolyte having an oxygen ion conductivity, and measure an oxygen partial pressure due to equilibrium reactions between the solute element (Y) and the oxide (YOa) by the principle of an oxygen concentration cell so as to obtain the activities of the solute element (Y).
However, problems thereabout arise that when the oxide (YOa) changes into a gas at a room temperature (e.g., CO, CO.sub.2, NO.sub.2, SO.sub.2, etc.), it cannot be coated on the outer surface of the solid electrolyte, or when the oxide (YOa) changes into the gas (e.g., P.sub.2 O.sub.5) at the measuring temperature, the coated layer fades away. On the other hand, it may be assumed as one of solving measures that the composite oxide (e.g., Ca.sub.3 (PO.sub.4).sub.2, CaCO.sub.3, CaSO.sub.4, etc.) is employed. However, even in the methods utilizing the composite oxides, there exist no suitable substances stable at high temperature, for example, suitable nitrates for a case of a nitrogen sensor, carbonates for a carbon sensor, or sulfates for a sulfur sensor. For using the sensor in steel making, such oxides or composite oxides are required which remain stable and solid at the temperature of at least 1600.degree. C.